The electrical circuits formed on printed circuit (PC) boards, like larger scale, conventional circuits, need protection against electrical overloads. This protection is typically provided by subminiature fuses that are physically secured to the PC board. Examples of such subminiature, surface-mounted fuses are disclosed in U.S. Pat. Nos. 5,166,656 and 5,552,757.
Various problems have been encountered in such subminiature surface-mount fuses. Specifically, predicting the location along the fusible link where the fuse will blow has been difficult. To solve this problem, manufacturers have added a diffusion bar to the fusible link. The diffusion bar is produced from a material which has a lower melting temperature than the material used to form the fusible link. This practice is described in the commonly assigned U.S. Pat. No. 5,552,757, which is incorporated herein by reference. As the fusible link reaches a normal operating temperature which coincides with the melting temperature of diffusion bar material, some percentage of the diffusion bar diffuses into the fusible link. This causes a eutectic reaction in the diffusion bar area of the fusible link thus lowering the melting temperature of that portion of the fusible link so that the fusible link selectively blows in that region.
Manufacturers have also encountered problems with the diffusion bar. For instance, as the diffusion bar reaches its melting temperature, rather than diffusing into the fusible link, it will liquify and roll along the fusible link, and the desirable eutectic reaction will be adversely affected. In other instances, while in the molten state, the diffusion bar can ballistically project itself from the fusible link at the operating temperature of the circuit. This decreases the amount of the material in the diffusion bar available for the eutectic reaction, and the probability of overheating is increased.
In many instances overheating may lead to charring of the substrate material. Additionally, the combination of the heated fusible link and the charred substrate heat the epoxy conformal coating of the fuse to its flash point, eventually igniting it. This is undesirable because it can destroy the circuit and cause other hazards.
The present invention was developed to solve these and other problems.